Semiconductor lasers have found many industrial and commercial applications in recent years. For example, lasers are used in telecommunications, in pickups for optically readable media used in CD players, CD ROM drives and DVD players, in medical imaging, and in video displays. However, previously known semiconductor lasers have a number of disadvantages. For example, traditional semiconductor lasers, such as ones used in CD players, emit light from the edge of a chip, making it necessary to cleave a wafer into chips and to package the chip before determining whether the laser functions properly. Other types of light sources, such as LEDs do not provide the performance needed for certain applications.
Vertical Cavity Surface Emitted Lasers (hereinafter “VCSELs”) have been developed to address the need for a more advanced, higher quality laser that can function well in a variety of applications. VCSELs combine the performance advantages of LEDs and of edge-emitting lasers at costs comparable to LEDs. VCSELs emit light vertically from the wafer surface, like LEDs, allowing for fabrication and testing, which is fully compatible with standard I.C. procedures and equipment. VCSELs have the additional advantage that they can be formed into arrays. In addition, VCSELs are much faster, more efficient, and produce a beam with a smaller divergence than do LEDs.
The VCSEL structure leads to a host of performance advantages over conventional semiconductor lasers.
1) small size
2) low power consumption
3) two-dimensional array capabilities
In contrast to conventional edge-emitting semiconductor lasers, the surface-emitting VCSEL has a symmetric Gaussian near-field, greatly simplifying coupling to optical elements or fibers. In addition, VCSEL technology allows the fabrication of two-dimensional laser arrays.
However, VCSELs suffer from a number of disadvantages. Their manufacture requires sophisticated and expensive microfabrication. Since single-pass gain in thin layer semiconductor lasers is low, VCSELs incorporate highly reflective dielectric stacks which are integrated into the laser as Bragg reflectors. These consist of alternating layers of dielectric material, which are grown using methods of molecular beam epitaxy (MBE). This ensures a close match of the atomic lattice structures of adjacent layers. Alternating atomically ordered layers of materials with different electronic characteristics are thereby produced. The interfaces between the layers must be digitally graded and doped to reduce the electrical resistance.
Much work has been done to improve the performance of VCSELs by increasing the number of layers and/or the dielectric difference between alternating layers. However, this approach makes the fabrication more expensive and difficult. There is also a limit to the number of layers determined by the absorption in these layers. While VCSELs can be manufactured in two-dimensional arrays, there has been great difficulty in achieving uniform structure over large areas. The materials used for VCSELs generally do not have the desired low absorption and high index contrast over a broad frequency range. In particular, it is difficult to achieve high reflectivity in the communication band around 1.5 microns. In addition, VCSELs cannot be tuned in frequency since their periods cannot be changed. In addition, an external device must be used to control the polarization of the light.
In recent years, chiral materials, such as cholesteric liquid crystals have been demonstrated and proposed in a variety of lasing and filtering applications to address common drawbacks of standard semiconductor devices such as VCSELs. For example, a commonly assigned U.S. Pat. No. 6,404,789 entitled “Chiral Laser Apparatus and Method,” discloses a chiral laser with a defect formed by a light-emitting material layer. While this approach is advantageous with respect to previously known techniques, it may be difficult to construct a layered structure having a precise light emitting material thickness required to produce a defect (the required thickness must be approximately equal to the wavelength of light in the medium divided by 4). More importantly, the position of the localized state caused by the defect cannot be easily controlled because the thickness of the light-emitting material cannot be changed once the device is manufactured.
One approach that addressed this problem was disclosed in the commonly assigned U.S. Pat. No. 6,396,859 entitled “Chiral Twist Laser and Filter Apparatus and Method” which is hereby incorporated by reference herein in its entirety. The novel approach of this patent involved creating a localized state by inducing a defect in a chiral structure composed of multiple chiral elements, by twisting one element of the chiral structure with respect to the other elements along a common longitudinal axis such that directors of the element's molecular layers that are in contact with one another are disposed at a particular “twist” angle therebetween. The resulting “chiral twist structure” enabled control of the position of the localized defect state within the photonic band gap by varying the twist angle.
This novel chiral twist structure is advantageous for a variety of applications including, but not limited to, EM filters, detectors, and lasers that are readily tunable by varying the twist angle. The only limitation of a chiral twist structure is the width of the photonic band gap within which the defect state may be moved. Essentially, the width of the photonic band gap determines the tunability bandwidth of the device. In certain telecommunication applications, it may be useful to have two or more defect states in the expanded band gap.
It would thus be desirable to provide a chiral structure and method of provision thereof that has a greater tunability bandwidth (i.e. an expanded reflection band) than a standard chiral twist structure. It would further be desirable to provide a chiral structure and method of construction thereof that comprises two or more independently controllable defects within the expanded reflection band.